1. Field of the Invention
This invention relates generally to the prevention of erythema of the skin caused by the absorption of burning ultraviolet radiation while concomitantly allowing tanning of the skin, and more particularly relates to sunscreen compositions having ideal properties for such purpose.
2. Description of the Prior Art
Extensive studies have been made of the ultraviolet radiation of sunlight and skylight reaching the surface of the earth and the effects of such radiation on the human skin. It has been established that the radiation between 295 and 315 nanometers produces substantially all of the erythemal energy and a substantial portion of the tanning energy with a maximum erythemal effect being demonstrated at 296.7 nanometers. Radiation between 315 and 400 nanometers, the near ultraviolet, is pigmentogenic but not erythemogenic, and promotes incident tanning. The differing intensities and the erythemal and tanning effectiveness of the various wavelengths within these ranges have been established and methods have been determined for calculating accurately their effects on normal untanned skin.
The only practical technique for preventing sunburn is to limit the exposure of the skin to the erythemogenic range of sunlight and skylight radiation to dosages less than those required to produce burning. This may be accomplished by avoiding exposure of the individual for more than the shortest periods of time. Such limitations do not, however, fit into the designs of a civilization which enjoys outdoor activity and admires melanization of the skin through insolation. Avoidance of exposure is not necessary because the effects of the exposure may be mitigated readily by the use of an ultraviolet screening agent which, applied to the skin, attenuates the dosage of erythemogenic radiation reaching the skin.
Products containing ultraviolet screening agents are generally evaluated on the basis of exposure to summer noon-time sun for a four-hour period, the total erythemal flux of such exposure being limited by the ultraviolet screen to provide the desired degree of protection and tanning with safety. To provide fast tanning with minimal protection, approximately 10-18% of the total erythemal flux of sunlight and skylight should reach the skin in the four-hour period. For regular protection and tanning, 6-12% of the total erythemal flux should reach the skin. For extra protection, only 1-6% of the total flux should reach the skin, while less than 1% should be allowed to reach the skin where a total blocking effect is desired.
Approximately 76% of the physiological tanning potential of sunlight is found in the ultraviolet region between 295 and 315 nanometers, the erythemal area. The balance is found in the range between 315 and 400 nanometers, the tanning area. The erythemal limitations necessarily control the amount of tanning which may be obtained from the ultraviolet in the erythemal area, but there is no limitation on the availability of tanning from the ultraviolet in the incident tanning area since there is no appreciable erythemal effect. To provide the desired amount of protection and/or tanning, then, ultraviolet screens can be utilized to furnish the required erythemal flux transmission by adjustment of the percentage of such screen used.
There are many other properties necessary for an ideal sunscreening agent besides having the proper ultraviolet absorption characteristics. These can be listed as follows:
(1) the screening agent should demonstrate a relatively sharp absorption cutoff at 315 nanometers so that it transmits the desired tanning radiation at wavelengths above the erythemal range; PA0 (2) the screening agent should show resistance to chemical and photochemical changes in structure which decrease its ultraviolet absorbant effectiveness; PA0 (3) the screening agent should not be easily absorbed through the skin or cause any pathological side effects such as primary irritation, inducing sensitivity or allergic response, should not impede melanogenesis, should not induce any trophic changes in the skin, and should not interfere with any normal growth or metabolic processes of the skin and mucous membranes and associated organs; PA0 (4) the screening agent should be relatively insoluble in water and perspiration, while being sufficiently oil and solvent soluble for versatility in formulating in a variety of vehicles and for stability in oil-water emulsions; PA0 (5) the screening agent should be of such a character that it is capable of forming continuous films on the skin when applied by itself or in a convenient carrier vehicle for uniform protective effect; PA0 (6) the screening agent should exhibit no odor or its odor should be easily masked so as not to interfere with the perfuming of a product; and PA0 (7) the screening agent should exhibit little residual staining to skin and clothing, especially in the presence of sunlight, heat, laundry detergents, perspiration and the like.
Sunscreen agents in common use fail to exhibit one or more of the desired properties set forth above. These agents consist of individual species or mixtures of the salicylates, para-aminobenzoates, cinnamates, naphthoates, gallates and benzophenones:
(a) The salicylates have the disadvantage of screening out a considerable portion of the rays in the tanning region, particularly in the 315-330 nanometer range. Additionally, they exhibit a low absorptive capacity which requires the use of high concentrations in order to be effective. Many salicylates exhibit a highly undesirable odor level and a pronounced analgesic and numbing effect when applied to the skin. Their oil solubility is also rather limited in most cases.
(b) The para-aminobenzoates also have the disadvantages of having an analgesic and numbing effect on application to the skin. Para-aminobenzoic acid, which has poor overall solubility, lends itself to alteration by esterification with the proper alcohol to produce solubilities of the desired nature to a limited extent. Such aminobenzoate esters, however, have the great disadvantage of imparting a high degree of yellow to brown permanent stain to fabrics, e.g. clothing, beachwear, and towels, especially in the presence of or on exposure to ultraviolet light. Additionally, high-melting aminobenzoates readily crystallize on the skin and neither form a continuous film nor adhere well enough to afford satisfactory protection. The aminobenzoates also demonstrate a gradual cutoff resulting in the screening out of rays in the tanning region, particularly between 315-340 nanometers.
(c) The naphthoates have poor solubility characteristics, an overly broad absorptive spectrum, and low absorptivities.
(d) The cinnamates demonstrate good absorptivities, good solubilities and exhibit little staining. Their absorption maxima, however, do not extend far enough into the erythemal range, while they exhibit a decided cutoff in the tanning region between 315-345 nanometers. Additionally, when the esters of cinnamic acid are derivatives of volatile alcohols, the resulting cinnamate tends to be an irritant and often has a high odor. Other substituted cinnamate esters produce a distinct sensation of warming around the nostrils and eyelids when first applied, a sensation which many users find unpleasant and which often presages irritation. Forming esters of high molecular weight or low volatility reduces these irritant/odor tendencies to a satisfactory level but also reduces the sun-screening efficiency by lowering the absorptivity.
(e) The benzophenones tend to crystallize readily on the skin, and neither form a continuous film nor adhere well enough to afford satisfactory protection, while benzalacetophenone demonstrates marked irritant effects. Additionally, the benzophenones demonstrate a wide absorption spectrum which includes not only the erythemogenic wavelengths but also the pigmentogenic or tanning wavelengths.
None of the sunscreens known to the art, then, provide the optimum combination of properties desirable for use in preventing erythema of the skin from exposure to sunlight and skylight while concomitantly affording tanning of the skin, all of said prior art screens exhibiting one or more undesirable properties for such use.